Cyanate baths



United States Patent "ice 3,194,696 CYANATE BATES Johannes lvliiller,Nen-l'senburg, Germany, assignor to Deutsehe GoldandSiiber-Scheideanstait vormals Roessler, ranltfnrt am Main, Germany NoDrawing. Filed June 11, 1963, Ser. No. 286,954 Claims priority,application germany, June 20, 1962,

2 7 2 Claims. (Ci. 148--28) Cyanide baths for hardening steel have beenknown. For example, it has been known that increased hardness of steelswith low or medium carbon content can be obtained by treating suchsteels in a cyanide-containing bath at temperatures of between 650 and1000 C. (1200. and 1840 F.). Usually such baths are made up of 15-95alkali cyanide, and a balance of chlorides and/or carbonates of alkaliesor earth-alkalies. Carbon and a small amount of nitrogen penetrate thesurface of the workpiece whereby the surface hardness is obtained by theformation of martensite resulting from a more or less severe cooling.Such cyanide-containing baths may, in certain cases, contain a maximumof 1% in cyanates.

This application relates to cyanate baths. For example, it has beenfound that surfaces with even better qualities can be obtained in a saltbath consisting of 3- 30% alkali cyanate, and a balance of chloridesand/or carbonates of alkalies or earth-alkalies. In this case, arelatively thin outer layer or zone of carbides and nitrides is formedon the steel surface (known as a compound layer) while beneath this zonea layer or zone of martensite is formed, causing a remarkable hardnessincrease on the surface of the workpiece. This outer thin compound layeror zone of carbides and nitrides is highly resistant to wear and seizureand, at the same time, the hard layer of martensite displays a highresistance to compressive stresses.

These qualities are particularly advantageous in the treatment of gears.

One important feature, in addition to those mentioned, is that due tothe absence of cyanide the bath is nonpoisonous and, therefore, specialprotective measures during operation and sludge removal are notrequired. The bath is a cyanate, not a cyanide, bath.

It is advisable to avoid or prevent decomposition of the cyanate. Thiscan be achieved by lining the bath container with a non-ferrous metal,such as titanium, nickel, cobalt, or their iron-free alloys. It isfurther recommended that other parts of the equipment, such as burnertubes, heating elements, etc., be built of the same metals.

The inevitable decomposition of cyanate takes place While carbonate isformed in the bath. Cyanide is formed in only insignificant quantities.The formation of martensite, resulting in the hardness increase,requires a more or less severe cooling from the operating temperature,depending upon the metal used.

REPLENISHMENT For replenishment purposes, salt mixes are used with ahigh percentage of cyanate, particularly above 40% or even pure cyanate.

Examples Example 1.A bath is made up of 10% NaCNO and 90% NaCl.

If a steel with a carbon content of 0.15% is treated in this bath at atemperature of 850 C. (.1560 F.) for 90 minutes, a compound layer orzone is formed, 20 microns in thickness, consisting of carbides andnitrides. Beneath this compound layer another layer is formed,

3,l9d,5% Eatented July 13, 1965 approximately 350 microns in thickness,penetrated by carbon and nitrogen, which after quenching, because of theformation of martensite, has a hardness of 800 Vickers-the originalhardness of the material having been 150 Vickers.

The following mix is used for replenishment: 50% NaCNO and 50% NaCl.

This mix is added in such an amount as to result in a cyanate content ofbetween 510% in the bath.

Example 2.A bath made up of 10% NaCNO, 50% NaCl, and 40% KCl.

A replenishment mix consisting of 50% NaCNO, 25% KCl, and 25 NaCl wasused.

Replenishment is made in amounts sufiicient to maintain a cyanatecontent of 510%.

Compared to bath #1, this melt has the advantage of a lower meltingpoint. It can be used under 800 C. 1620" F.) for the intended purpose.

Example 3.A bath is made up of 5% NaCNO, 50% BaCl and 45% KCl.

A replenishment mix of 40% NaCNO, 30% BaCl and 30% KCl was used.

As in Example 2, this mix has a lower melting point. During operation ofthis bath, alkali carbonate is formed by oxidation of the cyanate. Thisalkali carbonate may be added from the start if, for instance a bath ismade up of 20% NaCNO, 40% 'Na CO 30% KCl.

If a lower melting point is not required, the addition of chlorides maybe omitted and a mix may be made up containing 10 to 20% NaCNO and to NaCO Such a melt need only be replenishment with sodium cyanate orpotassium cyanate to maintain the cyanate content necessary for asuflicient nitrogen and carbon concentration.

To eliminate heat radiation and fuming of the bath, particularly attemperatures of above 850 C. (1560 F.) or 900 C. (1660 F.), it is commonto use a graphite or carbon cover.

The bath container may be heated externally by means of gas, oil, orelectricity, or internally by immersed electrodes. In the latter case itwould be necessary to build the electrodes of a material similar to thatused for the container, or to coat them With such a material.

Now having described the cyanate bath hereof, reference should be had tothe following claims.

I claim:

1. A fused bath for increasing the hardness of steel consisting of 3 to30% of alkali metal cyanate and the remainder at least one memberselected from the group consisting of alkali metal and alkaline earthmetal chlorides, carbonates and oxides.

2. A fused bath for increasing the hardness of steel consisting of 3 to30% of alkali metal cyanate and the remainder at least one memberselected from the group consisting of alkali metal and alkaline earthmetal chlorides, carbonate and oxides confined in a container having asurface of an iron free metal in contact with such fused bath selectedfrom the group consisting of titanium, zirconium, nickel, cobalt andtheir alloys.

References Cited by the Examiner UNITED STATES PATENTS 2,007,136 7/35Albrecht et al. 14828 X 2,927,875 3/60 Muller l48-15.5

OTHER REFERENCES Metals Handbook, published by the American Society forMetals, 1948 Edition, pp. 576, 577, 694.

BENJAMIN HENKIN, Primary Examiner.

WINSTON A. DOUGLAS, DAVID L. RECK,

Examiners.

1. A FUSED BATH FOR INCREASING THE HARDNESS OF STEEL CONSISTING OF 3 TO30% OF ALKALI METAL CYANATE AND THE REMAINDER AT LEAST ONE MEMBERSELECTED FROM THE GROUP CONSISTING OF ALKALI METAL AND ALKALINE EARTHMETAL CHLORIDES, CARBONATES AND OXIDES.